What is a species?

How do we determine whether two animals are of the same species? It is not enough to judge based on similar appearance: Chihuahuas and Dalmatians look vastly different but we consider these to be the same species (in case you’re wondering the crossbreed is a “Chimatian”). So where does the boundary fall? In his book, Systematics and the Origin of Species (1942), famed evolutionary biologist Ernst Mayr proposed the definitive criteria that are still used today: “groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups”.

An article published today with PLOS ONE describes the first reported creation of a synthetic species, a fruit fly that has been christened Drosophila synthetica. Author Eduardo Moreno of the University of Bern describes a combination of commonly used lab variants that result in a fly that is capable of producing fertile offspring with others that are genetically the same, but not with its wild-type predecessor, Drosophila melanogaster. D. synthetica has smaller, paler eyes compared to D. melanogaster, and its wings also differ. But is synthetica a different species from melanogaster?

The key may be in the phrase: “a group of … interbreeding natural populations”. Lions can famously interbreed with tigers, to produce ligers. There is at least one documented case of a liger that was coaxed into breeding with a lion to produce a rather unhealthy off-spring that grew to adulthood. The reproductive barrier is not complete, but lions and tigers are still considered separate species. The issue? Ligers do not exist in the wild because their habitats do not overlap. The potential for lions and tigers to interbreed has only been demonstrated in captivity. Moreno acknowledges that D. synthetica may not meet the Mayr definition, and specifically refers to it as a synthetic species, to distinguish it from a natural species. He cites two reasons for this: “not only because it has been created in the lab but also because it may never be able to survive outside that laboratory environment.” Regardless, it seems he will challenge our notions of what it means to be your own species.

isomorphismes says right that Do we actually understand the genetic reasons that prevent two beings from procreating? I.e., if I gave you two sequences of DNA strings, could you say if they would be able to produce an offspring?